Control circuitry for controlling rotational speed of a DC motor

ABSTRACT

A simple control circuitry is provided for controlling the rotational speed of a DC motor. The control circuitry includes a voltage reference component and a switching circuit for controlling the state of the voltage reference component. When the voltage of the DC motor and the voltage reference component is no higher than a predetermined voltage level of the voltage reference component, the switching circuit does not conduct, and the DC motor is accelerated according to a first operation mode. When the voltage of the DC motor and the voltage reference component is higher than a predetermined voltage level of the voltage reference component, the switching circuit conducts, and the DC motor is accelerated to a maximum speed according to a second operation mode.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention provides control circuitry for controlling the rotational speed of a DC motor, and more specifically, simple control circuitry that switches the operation mode of a DC motor.

[0003] 2. Related Art

[0004] The processing and spreading of vast amounts of electronic data has facilitated the rapid exchange of information and knowledge, accelerated technological development and enriched our lives. However, when processing large numbers of data transfers, the central processing unit (CPU) of a laptop computer, for example, is prone to overheating. Therefore, it is essential that devices like laptop computers have an excellent heat dissipation device with minimal power consumption to eliminate the problem of overheating.

[0005] Please refer to FIG. 1. FIG. 1 is a simple block diagram of a heat-dissipating process of a CPU 12 as performed by a prior heat dissipation device 10. As shown in FIG. 1, the heat dissipation device 10 includes a DC motor 14, a driving circuit 16 that is electrically connected to the DC motor 14, and a fan 18 electrically connected to the DC motor 14. When the heat dissipation device 10 dissipates heat from the CPU 12, the driving circuit 16 first transmits a rotation signal, usually a current signal, to control the rotation of the DC motor 14. Next, the fan 18 is turned on by the DC motor 14, thereby cooling the CPU 12. Generally, the fan 18 is directly attached to the DC motor 14, i.e. the rotational speed of the fan 18 is that of the DC motor 14. When heat generated by the CPU 12 increases, the current signal outputted from the driving circuit 16 increases gradually, and the rotational speed of the DC motor 14 and the fan 18 increases as well. When the heat generated by the CPU 12 is relatively small, such as when little data is being processed, the DC motor 14 is not required to turn as fast. That is, the driving circuit 16 outputs a smaller current to the DC motor 14, thus saving power.

[0006] For a common DC motor, its characteristics are set after the design stage. That is, the input voltage and the rotational speed of the motor are directly related. FIG. 2 is a graph of input voltage versus rotational speed of the common DC motor 14 according to the prior art. Suppose the relationship between the input voltage and rotational speed of the DC motor 14 is represented by a curve Ti; when the input to the DC motor 14 is 5 volts, the rotational speed of the DC motor 14 is 4000 rpm. When the input to the DC motor 14 is 2.5 volts, the rotational speed of the DC motor 14 is 200 rpm, which is too high given the circumstances (i.e. the input current is too large).

[0007] The coil windings inside the DC motor 14 can be redesigned in order to reduce the motor's rotational speed at low input voltages. For example, the DC motor 14 can be designed to operate at 1500 rpm for an input voltage of 2.5 volts. The input/output graph of the DC motor 14 then follows a new characteristic curve T2. However, though the goal of dropping the rotational speed at lower input voltages has been achieved by following the characteristic curve T2, the rotational speed at higher voltages has been greatly compromised. As shown in FIG. 2, the rotational speed at an input voltage of 5 volts is only 3500 rpm.

OBJECT OF THE INVENTION

[0008] In light of the above-mentioned issues, the object of the invention is to provide control circuitry for controlling the rotational speed of a DC motor. The invention achieves this object with a simple design, few electronic components and no modifications to the original design of the DC motor.

[0009] The specific details of the contents and techniques of the invention are described with figures hereinafter:

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention will become more fully understood from the detailed description given hereinafter. However, the drawings are for the purpose of illustration only, and thus are not limitative of the invention, wherein:

[0011]FIG. 1 is a simple block diagram of a heat-dissipating process of a CPU as performed by a prior heat dissipation device;

[0012]FIG. 2 is a graph of the relationship between the input voltage and rotational speed of the DC motor according to the prior art;

[0013]FIG. 3 is a circuit diagram of control circuitry applied to a DC motor according to the first embodiment of the present invention;

[0014]FIG. 4 is a graph of the relationship between the input voltage and rotational speed of the DC motor depicted in FIG. 3;

[0015]FIG. 5 is a circuit diagram of control circuitry applied to the DC motor depicted in FIG. 3 according to the second preferred embodiment of the present invention; and

[0016]FIG. 6 is a simple block diagram of control circuitry applied to the DC motor depicted in FIG. 3 according to the third preferred embodiment of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

[0017] Please refer to FIG. 3. FIG. 3 is a circuit diagram of control circuitry 20 applied to a DC motor 30 according to the first embodiment of the present invention. As shown in FIG. 3, the control circuitry 20 comprises an input node 21, which, along with the input node 31 of the DC motor 30, is connected to the variable voltage source 50, a voltage reference component 22 that is electrically connected to the input node 21, a switching circuit 24 that is connected between the DC motor 30 and the voltage reference component 22, and a ground node G.

[0018] The voltage reference component 22 of the present invention is a Zener diode ZD, which provides a predetermined reference voltage Vzd. Between the cathode of the Zener diode and the input node 21, there is usually a current-limiting resistor R1, which increases the resistance of the voltage reference component 22, effectively limiting the current through the Zener diode ZD and thus lengthening the life of the Zener diode.

[0019] The switching circuit 24 of the first embodiment of the invention is used to control the state of the voltage reference component 22 and is connected through the connector node 25 to the DC motor 30. The switching circuit 24 consists of an npn bipolar junction transistor (BJT) Tr and a voltage drop component. The base B of the transistor Tr is connected to the voltage reference component 22, and the collector C and the emitter E are connected to the DC motor 30 and the ground node G of the switching circuit 24 respectively. When the switching circuit 24 conducts, current generated by the variable voltage source 50 flows to the emitter E through either the DC motor 30 and collector C or the voltage reference component 22 and base B. Additionally, a resistor R2 connected across the collector C and the emitter E functions as the voltage drop component, thus ensures a voltage difference between the collector C and the emitter E. Since the purpose of the voltage drop component is to provide a voltage difference between the collector C and the emitter E of the transistor Tr of the switching circuit 24, the component can also be a Zener diode or any resistive component.

[0020] Please refer to FIGS. 2, 3, and 4. FIG. 4 is a graph of the relationship between the input voltage and rotational speed of the DC motor 30 of the first embodiment. The components of the control circuitry 20 described hereinafter shall assume values as described in FIG. 2 for ease of explanation of the operating principles. As shown in FIGS. 3 and 4, assume that the reference voltage Vzd of the Zener diode ZD is 3 volts and the DC motor 30 is designed to have the characteristic curve T2.

[0021] When the variable voltage source 50 outputs a voltage of 2.5 volts to the DC motor 30, the rotational speed of the DC motor 30 is 1500 rpm. At this point, the input voltage is still smaller than the reference voltage Vzd (3 volts) of the Zener diode ZD, so the npn BJT Tr of the switching circuit 24 does not conduct. Therefore, the current outputted by the variable current source 50 only flows to the ground node G through the DC motor 30 and resistor, and the motor operates according to the characteristic curve T2.

[0022] When the variable voltage source 50 outputs a voltage greater than the sum of the potential difference across the current limiting resistor R1 and the reference voltage Vzd of the Zener diode ZD, for example, 3.5 volts, the Zener diode conducts and consequently switches the npn BJT Tr on. The current generated from the variable voltage source 50 then mostly passes through the DC motor 30, the collector C of the BJT Tr, the emitter E of the BJT Tr, and the ground node G, while only a little amount of current flows through the Zener diode ZD and resistor R2. As a result, the DC motor 30 switches from the characteristic curve T2 to T1, and the motor reaches a rotational speed of 4000 rpm when the variable voltage source 50 provides a 5 volt input.

[0023] Please refer to FIG. 5. FIG. 5 is a circuit diagram of control circuitry 60 for controlling rotational speed of the DC motor 30 according to the second preferred embodiment of the present invention. The most significant difference between the first embodiment and the second is that the switching circuit of the second embodiment of the control circuitry adopts a pnp BJT. As shown in FIG. 5, the control circuitry 60 comprises an input node 61 connected to the variable voltage source 50, a resistor R3 connected to the input node 61 across which a voltage difference develops due to the current generated from the variable voltage source 50, a voltage reference component 22 electrically connected between the resistor R3 and ground node G, and a switching circuit 64 that is electrically connected between the input node 61 and the DC motor 30.

[0024] The voltage reference component 22 of the second embodiment also utilizes a Zener diode ZD to provide a predetermined reference voltage Vzd. There is also a current-limiting resistor R1 connected between the cathode of the Zener diode ZD and the input node 21 that boosts the resistance of the voltage reference component 22 so as to reduce the current flowing through the Zener diode ZD while it conducts, thereby lengthening the lifetime of the Zener diode ZD.

[0025] The switching circuit 64 of the second embodiment of the invention is used to control the state of the voltage reference component 22 and is connected through the connector node 65 at the node between the resistor R3 and the voltage reference component 22. The switching circuit 64 consists of a pnp bipolar junction transistor Tr and a voltage drop component. The base B of the transistor Tr is connected to the connector node 65, and the collector C and the emitter E are connected to the DC motor 30 and the input node 61 of the control circuitry 60 respectively. When the switching circuit 64 conducts, the current generated by the variable voltage source 50 flows to the DC motor 30 through either the collector C or emitter E of the pnp transistor Tr. Additionally, a resistor R2 connected across the collector C and the emitter E functions as a voltage drop component, thus ensures a voltage difference between the collector C and the emitter E. Since the purpose of the voltage drop component is to provide a voltage difference between the collector C and the emitter E of the transistor Tr of the switching circuit 24, the component can also be a Zener diode or any other component with resistive characteristics.

[0026] A driving circuit 70 drives the DC motor 30. The control circuitry 20 and 60 is used to modulate and control the rotational speed of the DC motor 30. The function and principles behind the control circuitry 20 and 60 have been thoroughly described in the first and second preferred embodiments of the invention, so no further explanations will be provided here.

[0027] The switching circuits 24 and 64 of the invention have only been embodied with npn and pnp BJTs Tr only. However, it is also possible to use a PMOS transistor, NMOS transistor, or a relay as a switch, all of which fall within the spirit of the invention, and no further details will be provided at this point. In addition, with reference to FIGS. 3 to 5, when the switching circuits 24 and 64 are not conducting, the characteristics of the DC motor 30 are affected, aside from its original design, by only the resistor R2. That is, when a user needs to lower the rotational speed of the DC motor 30 for a given voltage, a larger resistor R2 can be used to achieve this goal.

[0028] In contrast to the prior art, the most significant characteristic of the present invention is that the control circuitry 20 and 60 requires only the simplest electronic components to achieve the goal of modulating and controlling the rotational speed of a DC motor; no modifications to the internal windings of the motor or additional complex circuitry is required.

[0029] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A simple control circuitry, electrically connected to a DC motor, to modulate and control the rotational speed of the DC motor. The control circuitry comprises of, but is not limited to: an input node, which is connected with the input node of the DC motor to a variable voltage source; a voltage reference component, which is electrically connected to the input node of the control circuitry, and provides a predetermined reference voltage; and a switching circuit, electrically connected between the DC motor and the voltage reference component, controls the state of the voltage reference component. The switching circuit includes a ground node, and a connector node connected to the DC motor; wherein when the voltage provided by the variable voltage source to the DC motor and the voltage reference component is no higher than a predetermined voltage level of the voltage reference component, the switching circuit does not conduct, and the DC motor is accelerated according to a first operation mode; when the voltage of the DC motor and the voltage reference component is higher than the predetermined voltage level of the voltage reference component, the switching circuit conducts, and the DC motor is accelerated to a maximum speed according to a second operation mode.
 2. Circuitry according to claim 1, characterized in that the second operation mode of the DC motor operates at a higher rotational speed than the first operation mode for a given voltage.
 3. Circuitry according to claim 1, characterized in that the voltage reference component is a Zener diode.
 4. Circuitry according to claim 3, wherein said voltage reference component further comprises a current limiting resistor which is connected between the input node of the control circuit and the Zener diode, used to reduce the current through the Zener diode as a protective measure to the Zener diode.
 5. Circuitry according to claim 1, wherein said switching circuit is an npn bipolar junction transistor (BJT), an N-type metal oxide semiconductor (NMOS) transistor, or a relay.
 6. Circuitry according to claim 5, wherein said switching circuit further comprises a voltage drop component connected in parallel with the switching circuit, providing a voltage drop across switching circuit.
 7. Circuitry according to claim 6, wherein said voltage drop component is a resistor or a Zener diode.
 8. A simple control circuitry, electrically connected to a DC motor, used to modulate and control the rotational speed of the DC motor. The control circuitry comprises, but is not limited to: an input node, connected to a variable voltage source; a ground node; a first voltage drop component, which is electrically connected to the input node and develops a voltage drop when current generated from the variable voltage source flows through it; a voltage reference component, which is electrically connected to the first voltage drop component at one node, and the ground at the other, and provides a predetermined reference voltage; and a switching circuit, which is electrically connected between the input node and the DC motor, used to control the state of the voltage reference component, the switching circuit comprising of: a connector node, connected between the first voltage drop component and the voltage reference component, used to control the state of the voltage reference component; and a second voltage drop component, which is electrically connected between the input node of the control circuit and the DC motor, and provides a path for the current generated from the variable voltage source to the DC motor; wherein when the voltage provided by the variable voltage source to the switching circuit and the voltage reference component is higher than the voltage across the first voltage drop component and no larger than the predetermined voltage of the voltage reference component, the switching circuit does not conduct, and the DC motor accelerates according to a first operation mode; when the voltage provided by the variable voltage source to the switching circuit and the voltage reference component is higher than the predetermined voltage of the voltage reference component, the switching circuit conducts, and the DC motor accelerates according to a second operation mode to a maximum speed.
 9. Circuitry according to claim 8, characterized in that the second operation mode of the DC motor operates at a higher rotational speed than the first operation mode for a given voltage.
 10. Circuitry according to claim 8, wherein said first voltage drop component is a resistor.
 11. Circuitry according to claim 8, characterized in that the voltage reference component is a Zener diode.
 12. Circuitry according to claim 11, wherein said voltage reference component further comprises a current limiting resistor which is connected between the input node of the control circuit and the Zener diode, used to reduce the current through the Zener diode as a protective measure to the Zener diode.
 13. Circuitry according to claim 8, wherein said switching circuit is a pnp BJT, a P-type metal oxide semiconductor (PMOS) transistor, or a relay.
 14. Circuitry according to claim 13, wherein said second voltage drop component is a resistor or a Zener diode. 